This invention relates to a method for improving the selectivity of supported silver catalysts for the production of ethylene oxide. Specifically, the invention relates to an improvement in the known process for incorporating promoters into such catalysts.
Supported silver-based catalysts have been used industrially for many years for the oxidation of ethylene to ethylene oxide with oxygen or air. Most of the ethylene which is reacted is converted into ethylene oxide on the silver-impregnated catalyst support material and the remainder of the ethylene is converted almost exclusively to carbon dioxide and water. The goal is to react as much ethylene as possible, i.e., high productivity, such that the greater amount of the ethylene is converted to ethylene oxide, i.e., high selectivity.
It is well-known in the art that the incorporation of promoters, such as rubidium or cesium, into these catalysts will increase the selectivity thereof. U.S. Pat. No. 4,012,425, issued Mar. 15, 1977, discloses one such process which comprises treating the catalyst with a solution of cesium or rubidium. There are many similar disclosures in the art, both for the manufacture of new catalyst and for the regeneration of spent catalyst. However, nowhere in the prior art is there any disclosure that any particular anion should be used with the promoters, other than that the anion should not be a catalyst poison such as sulfur-containing compounds. The above patent states that no unusual effectiveness is observed with the use of any particular anion and goes on to say that nitrates, nitrites, chlorides, iodides, bromates, bicarbonates, oxalates, acetates, tartrates, lactates, and isopropoxides may be used.